Since the beginning of civilization, gemstones have always been thought to possess metaphysical powers. All the main ancient cultures, Vedic, Egyptian, Mayan, and Greek, had used these colorful and shiny pebbles for ritualistic, sacramental and healing purposes. The idea of using gemstones as solely item of adornment and wealth was a later development; yet, even this shift of perception, had originally an occult reason behind it.
Gems seems to have a special power, a transcendental quality that go beyond their colorful mirror, the thought of far away lands and mysterious places.
In ancient India, a number of Sanskrit texts on gems have been written.
Of a purely scientific, gemological knowledge are works like "Ratnapariksa" (literally 'Gemology'), and "Mani-Mala". More specifically, the "Graha-Gocara Jyautisha", the "Garuda Purana", the "Brhat Samhita", the "Agni Purana" are the texts dealing with planetary gemology.
As we already know, according to Vedic astrology, nine main gemstones are associated with nine planets.

Associated Gems:

with the sun is the RUBY,
with the moon is the PEARL,
with mars is the RED CORAL,
with mercury is the EMERALD,
with Jupiter is the YELLOW SAPPHIRE,
with Venus is the DIAMOND,
with Saturn is the BLUE SAPPHIRE,
with Rahu is the HESSONITE,
and with Ketu is the CAT’S EYE.

It is clearly stated in many passages of the different Vedic texts that "inward luster, transparency, illumination with rays, sparkle, free from impurities and good formation of the shape are the characteristics of good gems" (Agni Purana, 246.13-14).

"Gems are not good if they are sandy, cracked within, scratched, stained, if they are lusterless, rough, dull or mixed with mineral substances even though they may have all the characteristic features of their family" (Garuda Purana, 70.18).

"Since a jewel (gem) endowed with good characteristics ensure good luck, prosperity and success to kings, and one with bad ones, disaster and misfortune, connoisseurs ought to examine their fortune depending on jewels (gems)" (Brhat Samhita, 80.1-3).

"If anyone wears a gem of many flaws out of ignorance, then grief, anxiety, sickness, death, loss of wealth and other evils will torment him" (Garuda Purana, 70.19).

A special attention needs to be addressed to the always-included emeralds. References from the Sanskrit texts claim that emerald inclusions are accepted, as far as the inclusions are not detracting from the overall transparency and beauty of the gem.
"Experts in gems are inclined to say that the following types of emeralds are of good potency: Emerald that has dark green color or the color of a spring meadow, a soft glow, that is not highly complicated (referring to inclusions), hewn in different ways and appear to be stuffed with gold dust (still referring to very small inclusions); emerald that is endowed with qualities of shape, even shade all over, sufficient heaviness and shoots diffusion of light when sun's rays fall on it" (Garuda Purana, 71.12-15)

The use of alternative gemstones is practiced as well, especially for those gems which can be expensive over two carats sizes, such as diamonds and rubies. It is found, however that the alternative gem has a milder effect than the correspondent main gem. See in the table below the alternative gems, metals, and fingers used to wear rings related to the main gem and the correspondent planet.